The effect of genotype and some climatic factors related to the Crown disease/Common spear rot (CD/CSR) syndrome was studied in several trials in two regions of Costa Rica (Central and South Pacific).

Disease incidence and yield data of 1 to 6 year old palms were collected between 1986 and 1990. This data allowed the analysis of some aspects related to the genetic origin and to environment conditions. The periods of maximum incidence were found together with the effect on production of the symptoms: crown disease (CD), commom spear rot (CSR) and the CD/CSR syndrome.

Higher susceptibility was found in crosses within the same origin (AVROS x AVROS, Deli x Deli) than in crosses between different origins (Deli x AVROS, Deli x Ekona, Deli x Calabar). The same progenies planted in different localities showed different incidence which could be associated to environmental conditions. It was found that the appearance of the symptoms associated to CD/CSR syndrome reached the highest expression between 13 and 16 months after planting, and this corresponded to the beginning of the rainy season.

The relationship between symptom severity and yield suggests that the more damaging effect was CD as a main symptom. Early appearance of the disease was associated with a larger production decrease while its apperarance forty months after planting had little effect on production during the evaluation period.

Introduction

Crown Disease (CD) is the most frequent disorder affecting the normal growth pattern of oil palms during the early stages of their development. The bending of the rachis, from which the disease derives its name, and the total or partial destruction of the leaflets in the vicinity of the arching represent the classic symptoms of the disease (Kovachich,1957; Chinchilla 1987 and 1987b; Monge et al . 1992).

This disorder has been fully described by various authors (Kovachich 1957; De Berchoux and Gascon 1963; Turner and Gillbanks 1974) whose descriptions of the majority of the symptoms coincide. Soh (1969) stated that the disease was not lethal in itself, but that the palms become prone to common spear rot (CSR) and that furthermore, their potential for bunch production was adversely affected. Chinchilla (1987a) and Monge et al . (1992,1993) both mentioned that the symptoms of Common spear rot were associated with those of Crown disease.

Although the involvement of a particular biological agent in the development of this abnormality has been suggested (Turner and Gillbanks 1974; Monge et al . 1992) there is evidence that shows that susceptibility to CD/CSR is controlled genetically, and responds to the expression of a gene in its recessive form (De Berchoux and Gascon 1963). This mechanism was confirmed by Blaak (1970), who added that the existence of an inhibitor modifying the expression of the gene responsible for this process was also possible.

The hereditary mechanism suggested by the quoted authors, shows that the disease could be eliminated with relative ease by crossing resistant parents. However, not much work has been done in this area since the disease has been traditionally considered of little importance due to its temporary nature, and because its effect on bunch production is barely detectable once the plant recovers. In addition to this, both the responsible agent and the environmental conditions which are favorable to the occurrence of the condition, are unknown, a fact which has hindered the formulation of routine procedures to guarantee the identification of resistant parental lines.

This paper reports the incidence and severity of the disorder in several genotypes planted in two different environments, the periods of maximum incidence and the effect on yield of the CD/CSR on those materials.

Materials and Methods

The study was carried out between 1986 and 1992 in the experimental areas of the Oil Palm Research Program of ASD de Costa Rica situated in Coto and Quepos in the South Pacific and Central Pacific regions of Costa Rica respectively. The climatic conditions of both regions are outlined in Table 1 .

The assessment of the incidence of the CD/CSR syndrome was conducted from the first year of planting, at intervals of 3 to 4 months, and over a period of 8 to 64 months on those experiments planted between 1983 and 1990 in the Coto estate, and in 1987 in the Quepos estate. Using information taken from identical progenies, comparisons were made between the incidence in each region.

Based on symptoms, the plants were classified according to three categories:

• Plants with Crown disease (CD): those which at the time of assessment revealed the characteristic symptom of arching of the leaves. The plants in this category showed a variable, sometimes barely perceptible, degree of necrosis in their leaflets.
• Plants with Common spear rot (CSR): those showing extensive wet rot in the youngest leaves without the arching.
• Plants with Crown disease/Common spear rot (CD/CSR); plants in which both symptoms were clearly visible.

In the study, lines such as Deli, Ekona, Calabar, AVROS and Dami composite were used alongside introductions from the wild, chiefly Bamenda and Tanzania, as set out in Table 2 .

The information obtained was classified according to the age of the palms in order to determine the periods in which the disease occurred most frequently.

Lastly, in order to determine the effect of the disease on bunch production, information on CD/CSR from August 1986 to August 1990 taken form eight experiments planted in Coto in 1985 was used. In each experiment the number and weight of the bunches produced over a period of 36 months from the start of the harvest was recorded. This information was then related to the incidence of the disease according to its symptom category: CD, CSR or CD/CSR.

The diseased plants were further grouped in relation to the time of disease appearance, in the following way: early (14 to 23 months) intermediate (24 to 39) or late (40 to 63 months). The bunch production in each category was compared with that of healthy palms.

Results and Discussions

De Berchoux and Gascon (1963) found considerable variation in the level of susceptibility to CD/CSR by comparing planting materials of different origins; those of Deli dura showing clearly the greatest incidence, as opposed to the high degree of resistance shown by progenies of La Mé origin.

The variations in the degree of susceptibility in the genetic lines planted in Coto are outlined in Table 3 . Crosses within the same origin (intraorigin) showed a greater frequency of diseased palms. The three progenies of AVROS (BM119 x BM119) origin were the most susceptible with a 29.76% incidence; the degree of susceptibility was also high in progenies of Deli dura origin, 14.2% of the palms showing symptoms of CD/CSR.

In crosses between different origins (interorigin), incidence of the disease was lower. Only 0.86% of the palms of Deli x Calabar origin and 0.75% of Kigoma x AVROS were affected. The lines of most widespread commercial use such as Deli x AVROS and Deli x Ekona showed levels of incidence similar to each other, in both cases less than 5% ( Table 3 ).

Soh (1969) identified a greater incidence of CD in Deli dura progenies of Elmina origin in comparison with other subgroups of Deli dura origin such as Ulu Remis and Johore Labis. The progenies planted in Coto, coming from Deli x Deli crosses were regrouped according to their genealogy ( Table 4 ). Classified in such a way, palms which have some level of association with the progenitors BM20 and BM8, themselves originating from the Elmina group (Rosenquist 1985), showed a greater degree of susceptibility. Likewise, in the progenies Deli x AVROS, incidence increased in proportion to the greater representation of maternal progenitors BM8 and BM20 in the genealogic make-up of the tenera progenies under examination ( Table 5 ).

Table 6 shows the families that displayed a greater degree of incidence of CD/CSR. It can be seen that progenies of crosses of the same palm (eg. HC133:1286D) showed a high degree of incidence, which emphasizes the importance of the genetic factor in the nature of the disorder. These findings coincide with those of Soh (1969), De Berchoux and Gascon (1963) and Blaak (1970). Although it has been found that the same genotype may respond differently to the disorder under diverse environmental conditions (Breure and Soebagjo 1991; Monge et al . 1992). The crosses in Table 6 belong to experiments planted in different years, a fact which reduces the possibility of CD/CSR being merely the result of the oil palms reaction under specific conditions, and reaffirms the strong genetic factor in the development of the disorder.

Turner (1981), Breure and Soebagjo (1991) and Monge et al . (1992), highlighted inconsistencies in the degree of incidence showed by similar genotypes planted in different areas, and attributed these differences to the effect of the environment on the occurrence of the disease. This effect can be seen in Table 7 , where the progeny HC133:1254D x HC129:1047P (Deli x AVROS) was used as a test cross in several experiments. In the Coto area this progeny displayed an incidence varying between 0 and 28.60%, with an average of 13.09%; in Quepos the incidence was greater, varying between 2.70 and 27.7% and with an average of 20.66%.

It is possible that these differences were associated with the extreme water deficiency which the palms experienced in the Quepos area which can cause changes in its growth pattern. The commencement of the rains creates very favorable conditions for rapid plant growth, which has been recognized as an important predisposing factor to the disease Chinchilla (1987a, 1987b); Monge et al . (1992, 1993). In Coto, on the other hand, there is a more even distribution of rains, allowing sustained growth to continue almost the whole year round, which could explain the lower degree of incidence of the disease.

The data in Table 8 shows the behavior of another group of identical progenies assessed in each environment, once again revealing a greater incidence of the disease in Quepos.

The time of maximum incidence of CD/CSR occurred between 13 and 16 months after transplanting (Fig 1), as found by Breure and Soebagjo (1991) and Monge et al . (1992). The period of high sucesptibility coincided with certain growth stages such as the renewal of rapid growth after recuperation from the stress of transplantation, and the renewal of physiological activity in the plant once the rains started. This fact should be considered while formulating methods for selection of resistant plants.

The effect of CD/CSR syndrome on oil palm productivity has been documented. Breure and Soebagjo (1991) identified a considerable reduction in yield in palms with symptoms of CD. The said authors proposed that the bending of the leaves might affect their capacity for intercept ing light, and therefore the process of photosynthesis, resulting in a decrease in yield.

The negative effect the disease has on bunch production was confirmed by the behavior of the progenies assessed in Coto. Furthermore, there seems to be a differential effect on production in accordance with the predominant symptoms. When diseased and healthy palms were compared, it was seen that those plants in which only CD or the combination CD/CSR was present had a greater reduction in yield than when only the CSR symptoms were present ( Table 9 ).

In palms that developed CD between 14 and 23 months after transplanting, the beginning of production was delayed considerably; healthy palms of the same progenies were harvested after 24 months, in comparison with 36 months for affected palms ( Table 10 ). Among palms affected by CSR and CD/CSR, and showing the first signs of the disease between 14 and 23 months, there were similar levels of yield, although always slightly lower than with healthy plants; nevertheless, this difference was not as extreme as with palms affected by CD alone.

When the first symptoms of CSR and CD/CSR appeared between 24 and 39 months after field transplanting, accumulative yield from healthy and diseased plants was similar. However, when the predominant symptom was CD the accumulative yield from diseased plants was less than that of healthy plants from the start of the assessment stage, even before the appearance of the first signs of the disease ( Table 10 ).

Palms showing any of the symptom described (CD, CSR,CD/CSR), after 40 months, did not result in any significant reduction in productivity. This would seem to indicate that the reduction in the leaf area caused by the disease in a more advanced stage of the palms development does not reduce the plants capacity for photosynthesis or assimilation to such an extend that the potential for bunch production is affected ( Table 10 ).

Behavior of palms free of symptoms within susceptible progenies

Palms free of the disorder throughout the period of observation were assigned to one of four groups:

1. Progenies free of the symptoms.
2. Progenies where the CD symptoms predominated.
3. Predominance of CSR.
4. Predominance of the combination CD/CSR.

Healthy plants from progenies which only displayed CSR had greater yields, even higher than those progenies in which none of the symptoms could be detected. The productivity of healthy palms from crosses in which CD was present, was the lowest, and from those which showed the CD/CSR combination the productivity was moderate (Fig. 2).

Where families showed a high incidence of CSR, relatively high bunch production in healthy and diseased plants alike was evident, and the same was true where CD was predominant. Nevertheless, when both symptoms appeared simultaneously there resulted a drastic reduction in yield, a fact which suggests an additional negative effect caused by CD and CSR on productivity (Fig 2). These observations might be an indication that the three symptom types in question could be caused by different agents, or at least suggest the existence of differences in the genetic constitution of the progenies in which one or other expression of these is predominant. Such differences apparently affect not only the response of the genotype to the disease, but also other genetic expressions such as yield and plant growth characteristics.

The effect of these symptom types on productivity is an area which requires further investigation. Monge et al. (1992) relates the presence of different symptomatologies to the lignification process of the tissues, which in one way or another could be associated with conditions or growth potential, and with yield.

Conclusion

The confirmation of a hereditary factor in the phenotypic expression of Crown disease/Common spear rot, is one clear conclusion of the present study. Crosses within the same family (intraorigin) seemed to have a higher genetic frequency of those alleles related to the susceptibility to this disorder, this frequency being particular high in the Deli and BM119 families. These same results suggest a greater predisposition in the Deli dura progenies more closely related to palms originating from the Elmina station (BM8 and BM20).

The stage when the higher number of palms showed symptoms of CD/CSR was around 14 months after field transplanting. Another important factor which should be highlighted relates to the differences in infection shown by the same genotype under diverse environmental conditions.

There was a considerable decrease in individual bunch production in palms with symptoms, especially when these symptoms appeared in the early stages of growth. However it must be pointed out that owing to the low proportion of diseased plants in one plantation, alongside the temporary nature of the disease in most palms, the detrimental effect on cumulative yield per unit area is barely detectable.

It is evident that the selection of resistant parents is the most direct course to minimize the harmful effects of the CD/SCR. However, the effectiveness of the current selection procedures are limited by inconsistencies in the behavior of susceptible progenies, which under certain environmental conditions can behave like resistant genotypes, alongside the lack of knowledge about a specific biological agent associated with the syndrome.

It is important to continue the assessment of progenies according to their degree of susceptibility to the CD/CSR combination; although this objective must be combined with the formalization of new alternatives which, as well as the genetic factor, take into account the involvement of the environment and the physiological state of the plants, especially at the highest peaks of incidence.

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